Electromagnetic deflection yoke



May 19, 1953 1.5. BRYAN 2,639,314

ELECTROMAGNETIC DEFLECTION YOKE I Filed July 9, 1952 F7@ Z. f@ 0 T3 aff A, :u fzf Patented May 19, 1953 U NIT ED vSTATES PATENT OFFI 2,639,314

vELECTRQMAGNETIC DEFLECTION YOKE James S. Bryan, Philadelphia, Pa., assigner to Wilco-Corporation, Philadelphia, Pa., a corporation .of Pennsylvania Application July 9, 1952, Serial No. 297,896

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This invention relates to Aelectrical apparatus, and more particularly, to improved yokes for electromagnetically deflecting a beam 'of electrically charged particles. Such yokes 'are particularly adapted for use in television image transducers, in which 'they serve 'todeflect the electron beam of the Ypicture tube as determined b y applied horizontal and vertical scanning signals, and it is with respect Vto this use that the invention will be specifically described.

An important objective inthe design of ltelevision transducers is to 'produce van `electron beam having uniform focus throughout the scanning area of vthe cathode-ray tube. The need for maintaining a uniformly focussed beam throughout the image 'area is particularly apparent in the case of picture tubes lfor Aproducing color television images, .since beams exhibiting variations vin focus throughout the scanning area `produce erroneous changes in the saturation values of the colors reproduced by such tubes. The successful Yattainment of this objective has, however, become increasingly difficult in recent years because of the trend toward the use "of 4cathoderay tubes having screens which requires that the beam be vdeiiected through correspondingly increased angles in order to scan the entire image area.

In order to produce a beam which is uniformly focussed for all values of the deflection angle, it .is necessary that the beam lbe free from astigmatism `at all values -o'f this angle. An `astig- 'matic 'electron beam may be defined as a beam having Van elliptical loross-section and exhibiting iirst and second lines of focus. These two lines of focus are normal to the axis of the beam, are spaced apart along 'the said axis and lie in mutually perpendicular planes. As a general rule, at a point between `the rst and second lines of focus, the cross-section of the beam becomes a circle, known as the circle of least con-fusion. An astigmatic beam is generally considered to be focussed" when the circle of least confusion is coincident with the screen surface. It has been found that this 4circle of least confusion does not remain coincident with the surface of the screen throughout the scanning area, so that the beam spot changes in size as a function of the deflection angle. In the case of tubes having large screens with corrrespondingly large deflection angles, the spot size increases to such an extent that line image detail cannot be reproduced.

The astigmatism above mentioned is largely due to the electromagnetic systems heretofore used for ydeiiecting the beam. More particularly,

increasingly 'large viewing i.;

2 it has been found that the prior-art 'deiiectio'n yokes produce a magnetic ldeflection iield lwhich changes in yconfiguration along the axis oi 'the yoke as the 'ratio of the horizontal and 'vertical scanning signals applied to the yoke is varied. The configuration of a deiiecting eld may be defined with reference toa plurality of `secti'oning planes taken normal to 4the longitudinal axis of the deiiection yoke at regular intervals between the ends thereof and defining' a plurality of 'field areas, and a reference plane containing 'the said longitudinal axis and normal to each `oi 'the `sec tioning planes. It is found, in thefcase of priorart yolres, that the magnetic vectors contained in successive ield areas exhibit different lengths `and directions with respect to each other ffor diiferent ratios of the vertical and 'horizontal 'Jdevflection currents applied 'to the yoke. More particularly, during the scanning of the screen, at which time the ratio of the horizontal and vertical scanning currents changes, the vectors 'devlining the leld produced by lprior-art yokes Ldo not maintain the :same relative `magnitudes throughout the length of the field and do not maintain constant relative values as a function of their radial `position from the axis of the yoke. These effects are particularly pronounced iat the ends of the yoke where the problem of maintaining uniform direction and magnitude of the field along the axis of `the field and along the radius of the eld is made more severe because or' the fringing magnetic nelds existing at these regions. These deviations of the resultant magnetic field from geometric-similitude with rotation :and with radial and axial displacement, for changes in the :riore-mentioned current ratio, are primary factors imparting van astigmatic character to the beam.

It is accordingly an object of the invention to provide improved means for electromagnetically deiiecting a beam yof electrically charged particles.

.Another obj-ect of the invention is to provide improved electromagnetic yokes adapted to deflect an electron beam without imparting astigmatism to the beam.

A further object of the invention is to provide improved anastigrnatic electromagnetic deiiection yokes which are relatively simple and inexpensive.

Yet another object of the invention is to provide improved deflection yokes adapted to deflect electron beams over wide angles without imparting astigmatism to the beam, and which are particularly adapted for use in color television systems.

A specific object of the invention is to provide improved electromagnetic yokes in which the space-conguration of the magnetic lines of force produced thereby is substantially independent of the ratio of the values of the currents applied to the vertical coil system and the horizontal coil system thereof.

My invention is based on the discovery that, in order to achieve a yoke having horizontal and Vertical deflection coil systems adapted to deflect a beam in two dimensions over a screen surface without imparting astigmatic properties to the beam, it is necessary that the mutually perpendicular coil systems have substantially identical mean dimensions, not only along the axis of the yoke, but also radially from the latter axis. More particularly, I have found that a yoke structure achieving the above-noted objects may be produced by constructing the same so that the Iwindings of the mutually perpendicular coil syslend-turns of the mutually perpendicular coil systems conforms to a specified mean dimension. Furthermore, in order to achieve the desired anastigmatic characteristics, the axially extending conductors and the radially extending conductors of each coil system should exhibit a cosinusoidal distribution, and the arcuate conductors of each coil system should exhibit a sinusoidal distribution, of the ampere-turns.

Because of this novel construction, wherein extreme symmetry in the dimensions of the two coil systems is achieved, it has been found that the two magnetic elds produced are mutually perpendicular throughout the whole field volume, and the magnetic eld maintains the same space configuration irrespective of the ratio of the exciting currents.

The invention will be described in greater de-y `tail with reference to the appended drawings forming part of the specification, and in which: Figure 1 is a side view of a cathode-ray tube utilizing a yoke in accordance with the invention;

Figure 2 is a perspective View of one form. of yoke designed in accordance with the invention; and

Figure 3 is a section along the line 3-3 of Fig- I ure 2, taken normal to the longitudinal axis of the yoke.

The assembly shown in Figure 1 comprises a yoke II) constructed in accordance with the invention and positioned on the neck portion I2 of a cathode-ray tube I4 to produce horizontal and vertical deflection of the electron beam of the tube. The cathode-ray tube I4 itself may conform to `well known design and may comprise an electron beam-generating-and-accelerating gun assembly i 6 and a viewing screen I8, the latter element consisting of a layer of a luminescent,

tenebrescent or color-producing phosphor material. The yoke I fits coaxially about the neck `portion I2 of the tube and is preferably positioned so that one end thereof coincides with the plane of the junction 2l) of the neck and bulb portions I2 land 22 respectively.

By applying appropriate potentials to the elecf ing the so-called end-turns of the coils.

4 tron gun assembly I6 of the cathode-ray tube and to auxiliary :focussing means (not shown), a collimated electron beam 24 is produced having a direction substantially coincident with the longitudinal axis of the tube I4 and the deflection yoke I0. The electron beam 24, in passing through the vmagnetic iield established within the yoke and at .the entrance and exit portions of the yoke, is subjected to the desired deflection which, in accordance with the invention, takes place without concurrently imparting astigmatism to the beam. That is, in the process of deilection, cross-sections of the beam taken along its length are preserved substantially circular, and no distortion of the beam cross-section into the elliptical areas characteristic of astigmatism occurs. Because of this anastigmatic deflection, the minimum cross-section of the beam is not limited to that of a circle-of-least-confusion of an astigmatic beam,but may be focussed to a very ne spot at all lpoints of the raster, thus producing a uniformly sharp, high definition image on the cathode-ray tube screen.

In accordance with the invention, the deflec tion of the beam is produced, without imparting an astigmatic character thereto, by means of a yoke assembly I0 which produces a magnetic field exhibiting a fixed configuration during its rotation about the longitudinal axis of the yoke under the influence of the horizontal and vertical scanning currents applied tothe yoke. A specific form of the yoke Ill, according to the l.invention and adapted to producesuch a eld, is shown in greater detail in Figures 2 and 3. vThe yoke I0 comprises first and second coil systems arranged in mutually perpendicular planes toproduce mutually perpendicular deflection of the electron beam, and may further comprise a low reluctance return path forthe magnetic flux, such a path being provided by an enclosing cylinder of ferromagnetic material, e. g. of molded powdered iron.

The first of these coil systems, whichmay be utilized to deflect the beam in a horizontal direction, comprises first and second coil-pairs 54 and 56.- The coil-pair 54 consists of an outer coil 58 and an inner coil 60, whereas the second coil-pair 56 similarly consists of an outer coil 52 and Yan inner coil 64. l. Y. y

Each of the aforesaid coils consists of a multiplicity of parallel, axially arranged, insulated conductor portions which extend into loop-ends preferably bent at substantially right angles to the axially arranged conductor portions and form- In the present case, each coil is further shaped so that the axially extending conductor portions conform to a cylindrical surface of radius r, and, because of this configuration, the end-turns comprise first conductor portions in which the conductors extend substantially along a radius of the said cylindrical surface and second conductor portions in which the conductors are more or less arcuate.

Thus, the outer coil 58 of coil-pair 5,4 comprises parallel, axially extending'conductor portions 58d, 58e, radially extending conductor portions 58j, 58g, and arcuate conductor portions 58h, while the inner coil 60 comprises axially extending conductor portions 60d, 60e, radially extending conductor portions 60j, 60g, and arcuate conductor portions Ih. Similarly, the outer coil 62 of coilpair 56 comprises axially extending conductor portions 62d, 62e, radially extending conductor Z5' portions 621, 62g, and arcuate conductor portions 62h, While the inner coil 64 compriseslaxially extending conductor portions 64d, 84e, radially extending conductor portions 64j, 64g, and arcuate conductor portions 64h.

It is noted that the axially arranged conductor portions are symmetrically oriented with respect to a vertical axis 68 and are arranged .about the cylindrical surface of radius r. Furthermore, the end-turns of the inner coils, 60 and 84 are so dimensioned that the arcuate conductor portions 60h, and 64h t within the arcuate conductor portions 58hl and 62h of the outer coils 58 and 62 respectively, and are coplanar therewith.

The ratio of the number of axially extending conductor portions N1 in the outer coils 58 and B2 to the number of axially extending yconductor portions N2 of the inner coils 60 and 64 of the coilpairs 54 and 53 is adjusted so as to produce an ampere-turn distribution varying approximately as the cosine of the angle 6, where is measured with respect to an axis 'le normal to axis 68. The manner of calculating this ratio of turns to produce the desired ampere-turn distribution is well-A known to those skilled in the art and, in a spe-4 conductor portions 62d, 6M, 64e and 62e of coil-I pair 5B are located at corresponding diarnetricallyy opposite angle values. Because of the manner in which the coils of coil-pairs 54 and 5B are shaped, and because of the space distribution of the axially extending conductor portions, the radially extending conductor portions are also charac-' terized by an approximately cosinusoidal ampereturn distribution, whereas the distribution of the arcuate conductor portions approximates a sinusoidal ampere-turn distribution.

The second of the coil systems, which in the specic embodiment herein described is utilized to deflect the beam in a vertical direc-tion, comprises two coil assemblies, the nrs-t of which is constituted by two coil-pairs 12 and 14, and the second of which is constituted by two coil-pairs 18 and 18.

vThe coil-pair 12 comprises an cuter coil 80 and an inner coil 82. The outer coil 80 comprises axially extending conductor portions Bild, 80e, radially extending conductor portions 80j, 30g, and karcuate conductor portions 30h, while the inner coil 82 comprises axially extending v`conducaxially extending conductor portions 83d, 88e,Y radially extending conductor portions 88j, 68g,

and arcuate conductor portions 88h, while the inner coil 99 comprises axially extending conductor portions Std, e, radially extending conductor por-tions lf, Mg, and arcuate conductor portions 90h. The coil-pair 18 comprises an outer coil 92 andan inner coil 94. The outer coil .92 comprises v axially .extending conductor portions 92d, 92e-, radially extending conductor portions B2i, 92g, and arcuate conductor portions 92h, while the inner coil 94 comprises axially extending conductor portions 34d, 94e, radially extending conductor portions 9M, 94g, and arcuate conductor portions 94h.

It will be noted that the axially extending conductor portions of coils and '8.2 overlie the corresponding axially extending .conductor -portions of coils 84 and 86, and that the axially extending kconductor portions .of coils 88 and 90 ovcrlie the corresponding axially extending iconductor portions of coils 92 and 94. As in the case of the horizontal deflection coil system above described, the axially extending conductor `portions and the radially .extending conductor portions of the vertical deflection .coil system are characterized by an approximately cosinusoidal ampere-turn distribution, and similarly the arcuately arranged conductor portions are characterized by an yapproximately sinusoidal ampere-turn distribution. The ratio of the number of axially extending conductor portions N1 in the outer coils 89 and .84 .of coil-pairs 12 and 14 to the number of axially extending yconductor portions N2 of the inner 4coils 82 and 86, is adjusted so as to produce the abovenoted .ampere-turn distributions in the rst coil assembly. In the speciiic lcase shown, this ratio may be set at 1.1.3 when the axially extending conductor portions d, 84d; BM, tEd; 821e., 86e, and 580e, 84e are located at values of the angle 6 of substantially 101,25*, 146.25, 213.75 .and 258.75o respectively.

The rcoil-pairs 1.6 and 18 of the second coil assembly are mirror images of the coil-pairs 12 :and 1.4 respectively, and these coils accordingly conform to the geometry above described;

In accordance with the invention, all of Vthe mean dimensions of the horizontal deflection coil .system are made substantially identical to the mean dimensions of the vertical deflection coil system. More particularly, and as appears in Figure 3, Ythe average distance, from the longi tudinal axis 52 `of the yoke, .of the axially extending conductor portions 58d, 58e, 80d, 50e., Md, 62e, and 84d, 64e of the horizontal deflection coil system, is made equal to the average distance from this axis .of the axially extending conductor portions 30d, Md; 82d, 36d; 82e., 36e; 80e, 84e; 88e, 92e; 85e, 94e; 30rd, 94d and 88d, 92d of the vertical vdeflection coil system, notwithstanding the fact that .the `vertical deiiection coil system comprises twice as many coil elements as the horizontal deflection coil system. This is achieved by means of the overlaying of the coils as above described. The mean lengths of the axially extending conductor portions of the Vertical and horizontal coil deiiection systems are also made substantially equal by reason of the novel construction shown in Figure 2 whereby the coilpairs 12 and 16, and the coil-pairs 14 and 'L8 are arranged so that a plane preferably normal to the longitudinal axis 52 and contain-ing the endturns of the coil-pairs 54 and 55, lies equidistant between corresponding planes containing coilpairs 12 and 1B, and coil-pairs 14 and 18. Thus the mean dimension a, between the .end-turns of the horizontal deiiection system, is substantially equal to (b-l-c) /2, where b is the mean dimension `between the end-turns Aof the coil-pairs 12, 1E, and c is the mean dimension between the endturns of the coil-pairs 14, 18.

Furthermore, the average length of the lradially extending conductor portions 583, 58g, Blij, 60g, 62), 62g, and 64j, 64g of the horizontal deflection coil system is equal to the average length of the radially extending conductor portions f, 80g, 82f, 82g, 84j, 84g, 86j, 86g, 88j, 88g, 90j, 90g, 923, 92g, and 94j, 94g, of the vertical deflection coil system, so that the mean radius r1 of the endturns of the horizontal deflection coil system is equal to the average of the mean radii r2 and r3 of the end-turns of the vertical deflection coil system, i. e., 1'1:(r2-|r3)/ 2.

Because of the substantially complete identity of the corresponding mean dimensions of the windings of the horizontal and vertical coil systems, not only as to the axial length of the respective coils but also as to the radial dimensions of the end-turns thereof, a uniform magnetic field is produced by the windings which exhibits a substantially fixed configuration irrespective of the rotational position of the deflection eld determined by the relative magnitudes of the currents supplied to the space-quadratured windings. Consequently, since there is no change in the configuration of the uniform magnetic field with rotation thereof, the initially anastigmatic beam entering the deflection eld remains anastigmatic throughout all of the deflection angles of the beam.

While the invention has been described lin connection with the specific form of the deflection yoke shown in Figures 2 and 3, it is apparent that alternative constructions may be used. More particularly, while the mean radius r3 of the arcuate conductor portions of coil-pair it is shown in Figure 3 to be greater than 1'1, which in turn is greater than r2, it is well understood that all of these mean radii may be made equal, or of increasing value in the opposite sense, as long as the condition r1:(r2-l-r3)/2 isfulfilled.

Inv addition, while the end-turns have been shown in the preferred embodiment of the invention to be positioned in planes perpendicular to the longitudinal axis 52 of the yoke, these endturns may be positioned in planes forming an acute angle with the longitudinal axis.

Furthermore, while the assembly of coil-pairs 54 and 5B has been shown as constituting the horizontal deflection coil system of the yoke, and the assembly of coil-pairs 12, 14, 16 and 18 has been shown as constituting the vertical deflection coil system of the yoke, the respective assemblies may be interchanged in their functions.

' Moreover, while each of the coil-pairsv of the defiection yoke of the invention is set out above as a lumped-coil approximation for producing a cosinusoidal distribution of ampere-turns of the axially extending conductor portions and corresponding ampere-turns distributions of thev radially extending and arcuate conductor portions, va distributed winding having a cosinusoidal distribution of ampere-turns and having the endturns of one of the ycoil systems divided as abovedescribed, is equally feasible.

Additionally, while, in the specific arrangement shown, one of the coil systems is divided into two sections in order to obtain the desired interleaving of the end-turns of the two coil systems, it is evident that this coil system may be divided into (n+1) sections in order to achieve interleaving with a coil system divided into n sections.

Also, while the axially extending conductor portions of the respective coil systems have been shown as defining a right circular cylinder, it is apparent that this construction may be modified so that these conductor portions define a flared Lil) 8 yoke of the sort disclosed in U. S. Patent 2,570,425 of Carlo V. Bocciarelli, granted October 9, 1951, to the assignee of the present invention, when such a geometry is desirable.

While I have described my invention by means of specific examples and in a specific embodiment, I do not wish to be limited thereto, for obvious modifications will occur lto those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

1. An electron beam-deflecting yoke assembly 'comprising first and second coil systems for producing magnetic lines of force respectively in first and second mutually 4perpendicular directions, vsaid lines of force defining a magnetic field extending along the longitudinal axis of the said yoke assembly, said axis being substantially perpendicular to the said first and second directions, the said coil systems defining a surface of revolution about the said longitudinal axis, said first lcoil system comprising first and second coil assemblies of similar geometry, each of the said coil assemblies comprising a first coil having a multiplicity ofv axially arranged conductor portions conforming to the said surface of revolution, said conductor Iportions extending into opposing loop-ends each bent away from the said longitudinal axis and arranged respectively in rst and second planes perpendicular to a third .plane containing the said -longitudinal axis and a line parall-el to the said first direction, each of the said loop-ends having two groups of ran dially extending conductor portions, and arcuate conductor -portions interconnecting the said radially extending conductor portions, the said second coil system comprising third and fourth coil assemblies of similar geometry arranged in space-qua'drature with respect to the said first and second coil assemblies, each of the said third and fourth coil assemblies comprising second and third coils, each having a multiplicity of axially arranged conductor portions conforming to the said surface of revolution, said axially arranged conductor portions extending into opposing loopends each bent away from the said longitudinal axis, the loop-ends of the said second coil extending beyond the loop-ends of the said first coil and being arranged respectively in fourth and fifth planes perpendicular to a sixth plane containing the said longitudinal axis and a line parallel to the said second direction, an-d the loop-ends of the said third coil extending within the loop-ends of the said first coil and being arranged respectively in seventh and eighth planes perpendicular to the said sixth plane, each of said loop-ends of each of the said coils having two groups of radially extending conductor portions, fand arcuate conductor portions interconnecting the said radially extending conductor portions, the axially arranged conductor portions of the said second and third coils occupying radially aligned circumferentially spaced positions about ythe periphery of the said surface of revolution, the mean lengths of the said axially arranged and radially extending conductor portions and the mean radius of the said arcuate conductor portions of the said first coil assembly being substantially equal respectively to the mean lengths of the said axially arranged and radially extending conductor portions and the mean radius of the said arcuate conductor portions of the said second coil assembly.

f 2. An electron beam-deecting yoke assembly according to cla-im l in which thesaid axially arranged and radially extending conductor portions of the said first coil of the said first and second coil assemblies have ampere-turns distribu-tions approximately proportional to the cosine of an yangle measured with respect to the said second vdirection and the said arcuate conductor portions of the said first coil have an ampere-turns distrilbution approximately proportional to the sine of the said angle, and in which the said axially arranged and radially extending conductor portions of Ithe said second and third -coils of the said third and fourth coil assemblies have ampere-turns distributions approximately proportional to the vsine of the said angle, and the said arcuate conductor portions of the said second and third coils have an ampere-turns distrib u-tion approximately proportional to the cosine of the said angle.

3. An electron beam-deiiecting yoke assembly according to claim l in which the said surface of revolution about the said longitudinal axis is a circular cylinder.

4. A11 electron beam-deilecting yoke assembly according to claim 1 in which the said loop-ends of the said coils of the said first coil assembly are arranged in a rst given number of planes and the said loop-ends of the said coils of the said vthird coil assembly are arranged in a second given number of planes interleaved with the said planes of the said loop-ends of the said iirst coil assembly, the said second given number being two greater` than the said first given number.

5. An electron beam-deflecting yoke assembly according to claim 4 in which the said drs't and second coil assemblies each comprise one coil-pair and in which the said third and fourth coil assemblies each comprise two coil-pairs.

6. An electron beam-defiecting yoke asembly according to claim 1 in which the said first, second, fourth, iifth, seventh and eighth planes are each perpendicular to the said longitudinal axis.

7. An electron beam-deflecting yoke assembly according to claim l in which the mean radius of the said arcuate conductor portions of the said third coil is greater than the mean radius of the said arcuate conductor portions of the said rst coil, and the latter said mean radius is greater than the mean radius of the said arcuate conductor portions of the said second coil.

8. .fin electron beam-deflecting yoke assembly comprising iirst and second coil systems for producing magnetic lines of force respectively in iirst and second mutually perpendicular directions, said lines of force dening a magnetic eld extending along the longitudinal axis of the said yoke assembly, said axis being substantially perpendicular to the said rst and second directions, the said coil systems defining a circular cylinder about the said longitudinal axis, said rst coil system comprising rst and second coil assemblies of similar geometry, each of the said coil assemblies comprising a nrst coil-pair having an outer coil and an inner coil, each of the said coils comprising a multiplicity of axially arranged conductor portions conforming to the surface of the said circular cylinder, said conductor portions extending` into opposing loop-ends each bent away from the said longitudinal axis and arranged respectively in first and second planes substantially perpendicular to the said longitudinal axis, each of the said loop-ends of each of the said coils having two groups of radially extending conductor portions, and arcuate conductor portions interconnecting the radially extending conductor portions, the loop-ends of the said inner coil fitting within and being coplanar with the corresponding loop-ends of the said outer coil, said axially arranged and radially extending conductor portions of each of the said rst coilpairs having ampere-turns distributions approximately proportional to the cosine of an angle measured with respect to the said second direction and the said arcuate conductor portions having an ampere-turns distribution approximately proportional to the sine of the said angle, the said second coil systemcomprising third and fourth coil assemblies or similar geometry arranged in spaced-quadrature with respect to the said rst and second coil assemblies, each ci the said third rand fourth coil assemblies comprising second and third coil-pairs, each comprising an outer coil and an inner coil, each of said coils having a multiplicity oi' axially arranged conductor portions conforming to the surface of the said circular cylinder, said axially arranged conductor portions extending into opposing loop-ends each bent away from the said longitudinal axis, the loop-ends of the said outer and inner coils of the said second coil-pair being arranged in third and fourth planes perpendicular to the said longitudinal axis and extending beyond the said loopends of the said first coil-pair, and the loop-ends of the said outer and inner coils of the said third coil-pair being arranged in fifth and sixth planes perpendicular to the said longitudinal axis and extending within the said loop-ends of the said first coil-pair, the said nrst plane lying equidistant between the said third and fifth planes, and the said second plane lying equidistant between the said fourth and sixth planes, each of the said loop-ends of cach of the said coils of the said second and third coil-pairs having two groups of radially extending conductor portions, and arcuate conductor portions interconnecting the said radially extending conductor portions, the said axially arranged and radially extending conductor portions of the said second and third coilpairs having ampere-turns distributions approximately proportional to the sine of the said angle and the said arcuate conductor portions having an ampere-turns distribution approximately proportional to the cosine of the said angle, the said axially arranged conductor portions of the said first coil-pair occupying circumferentially spaced positions about the said surface of the said circular cylinder, being symmetrically disposed With respect to a seventh plane defined by the said longitudinal axis and line parallel to the said iirst direction, the corresponding axially arranged conductor portions of the said second and third coil-pairs occupying radially aligned circumferentially spaced positions about the said circular cylinder, said latter positions being interspersed between the said circumferentially spaced positions occupied by the said axially arranged conductor portions of the said rst coil-pair and being symmetrically disposed with respect to an eighth plane dened by the said longitudinal axis and a line parallel to the said second direction, the mean radius of the said arcuate conductor portions of the said first coil-pair being substantially equal to the average of the mean radii of .the arcuate conductor portions of the said second and third coil-pairs respectively, the lastnamed mean radius being greater than the firstnamed mean radius, said latter mean radius being greater than the said second-named mean radius, and a circular cylindrical band of ferromagnetic material enclosing the said axially ar- 11 ranged conductors of the said first and second coil systems.

9. An electron beam-deecting yoke assembly according to claim 8 in which the said axially arranged conductor portions of the two coil systems form sixteen groups spaced upon the said surface of the said circular cylinder at equal `values of the said angle, and in which the ratio -the number of axially arranged conductor portions of the said outer coils of the said second fand third coil-pairs to the number oi" axially ar- References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,228,821 Hansen Jan. 14, 1941 2,240,606 Bobb May 6, 1941 2,562,394 Schlesinger July 31, 1951 2,573,017 Haworth Oct. 30, 1951 2,578,343 Ekvall Dec. 11, 1951 2,605,433 Friend July 29, 1952 

